Curing concrete

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Concrete reaches over 90% of it's max strength in about 7 days. It takes 28 days to reach full max strength. Since the 90% value is most likely good enough (most designs have a safety factor) the 7 day period is often used as time frame to keep it wet and to protect it from high loads. Actually damp is as good or better than wet, especially for the first few hours, because you only need to prevent the drying action. As another poster said - cover with plastic sheets to keep it wet as it would dry in a few hours if uncovered.
Spraying it with curing compound helps a lot. It doesn't eliminate the need to keep the surface damp but it helps a lot with surface hardness which can avoid spauling. If its more than a few hours old it is too late to get much benefit from curing compound so don't bother then.
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Rather guess or spread misinformation.......
PCA - Portland Cement Association is a great source of accurate information
http://www.cement.org/basics/concretebasics_faqs.asp
U of Texas - Austin has done a great deal of research on concrete

fyi
1) 28 days is not maximum strength merely a commonly agreed upon point in time to test concrete strength (as are 3 day & 7 day tests) 2) most concrete mix designs under "normal curing conditions" are not a 90% max strength in 7 days........ more like 75% of 28 day strength
Once concrete is surfaced cured sufficiently to avoid spray damage, keep it wet (don't let surface dry out) for a few days or ideally several days (longer is better). Cover with plastic, burlap, carpet or spray on curing compounds. Wetter is better as is longer.
Water applied to surface or cure compounds are to ensure that sufficient water is present in the curing concrete to allow for complete hydration of the cement reaction.
Applied water replaces water lost to evap, soil or forms. Compounds are supposed to keep evap down.
cheers Bob
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